1. Field of the Invention
The present invention relates to a canned motor which can be assembled in a casing together with a pump that can be operated by the canned motor, and more particularly to a canned motor rotatable at a high speed of at least 10000 rpm, for example, in a high-temperature, high-pressure atmosphere, e.g., at a temperature of 400.degree. C. or higher under a pressure of 200 kgf/cm.sup.2 or higher.
2. Description of the Prior Art
One conventional canned motor having a pressure-resistant structure is generally constructed as shown in FIG. 1 of the accompanying drawings.
As shown in FIG. 1, the canned motor primarily comprises a rotor 11 mounted on a rotatable shaft 10 and a stator 13 fixedly disposed around the rotor 11 with a radial gap defined between the rotor 11 and the stator 13. A pump impeller- (not shown) is mounted on an axial extension of the shaft 10, which is rotatably supported by magnetic bearings.
The rotor 11 comprises a rotor core 21 mounted on the shaft 10 and having an outer circumferential surface covered with a rotor can 25. The stator 13 comprises a stator core 14 and windings extending through the stator core 14 and having coil ends 15 projecting from axially opposite sides of the stator core 14. The stator 13 is fixedly mounted in a casing 12 and has an inner circumferential surface covered with a stator can 18 which is fixed at its axially opposite ends to end plates 16 of the casing 12. The stator can 18 is spaced radially outwardly from the rotor can 25 by a small gap.
When the windings of the stator 13 are energized, they generate revolving magnetic fields for rotating the rotor 11 with the shaft 10.
Cylindrical coil end stiffener tubes 17 are axially disposed between the end faces of the stator core 14 and the end plates 16 of the casing 12. The coil end stiffener tubes 17 have an inside diameter which is the same as the inside diameter of the stator core 14. The stator can 18 covers the coil end stiffener tubes 17 and the inner circumferential surface of the stator core 14. The stator can 18 serves to prevent a fluid handled by the pump from entering the stator 13.
The rotor 11 comprises of a rotor core 21 having many coil slot bars, end rings 24, and balance rings 23 at the both sides thereof. The circumferential surface of the rotor 11 is covered by rotor can 25 for preventing a fluid front entering the rotor 11.
When an alternating current is supplied to the windings of the stator 13, the rotor 11 and the shaft 10 rotate in unison with each other, and the pump impeller connected to the shaft 10 rotates to pump the fluid. At the same time, part of the fluid is introduced into the gap between the rotor 11 and the stator 13 which are covered with the respective rotor and stator cans 25, 18 for cooling the rotor 11 and the stator 13.
The rotor and stator cans 25, 18 are formed by cutting sheets of metal such as nickel alloy each having a thickness ranging from 0.2 to 1 mm to suitable lengths, rounding the cut sheets to cylindrical shape, and then welding the butted ends of the rounded sheets. When the rotor 11 is assembled, the rotor can 25 is fitted over the outer circumferential surface of the rotor core 21. Therefore, it is necessary that the inside diameter of the rotor can 25 be slightly greater than the outside diameter of the rotor core 21. Since it is generally difficult to establish the inside diameter of the rotor can 25 highly accurately, it has been customary to cut the sheet of nickel alloy to such a length that the inside diameter of the rotor can 25 will be slightly greater than the outside diameter of the rotor core 21, i.e., a gap will be created between the rotor can 25 and the rotor core 21, allowing the rotor can 25 to be easily be mounted on the rotor core 21.
However, when the rotor 11 is used under a high pressure while a gap is present between the rotor can 25 and the rotor core 21, the rotor can 25 is brought into intimate contact with the outer circumferential surface of the rotor core 21. As shown in FIG. 2 of the accompanying drawings, the rot-or can 25 shrinks over the rotor core 21 and produces a radially outward wrinkle 26 which extends substantially the full length of the rotor can 25.
The radially outward wrinkle 26 on the rotor can 25 tends to cause a turbulence in the fluid in the gap between the rotor 11 and the stator 13, resulting in an energy loss and a reduction in the efficiency of the canned motor when it operates at a high speed. Furthermore, the radially outward wrinkle 26 may possibly be forced into contact with -the inner circumferential surface of the stator can 18, damaging the stator can 18.